1. Field of the Invention
The present invention relates to a post-processing device for applying a post-process to one or more sheets fed thereinto from a device of a preceding stage, and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus for printing an image onto paper (sheet), e.g., printer, copier, or Multi-functional Peripheral (MFP, or, multi-functional device), is provided with a paper containing portion in which lots of sheets of paper are loadable and from which the paper is supplied. Such an image forming apparatus supplies the sheets of paper one by one from the paper containing portion, conveys the sheets of paper, and prints an image thereonto.
In most cases, the image forming apparatus is internally or externally provided with a post-processing device for applying a post-process to paper on which an image has been printed. The post-processing device (finisher) applies a post-process called “alignment process” of aligning accumulated sheets of paper fed into a holding tray after the printing. Some post-processing devices are provided with a stapler for stapling a sheet bundle that has been subjected to the alignment process, a punching mechanism for punching a hole in the sheets that have been subjected to the alignment process, and so on.
In an image forming apparatus having such a post-processing device, the problem is how to operate the post-processing device when a plurality of sheets of paper having different sizes is used.
There has been disclosed a technology in which a start time of alignment operation is set to a late time for a sheet having a large size, and the start time is set to an early time for a sheet having a small size. According to the technology, alignment of sheets having a large size is secured, and at the same time, the productivity of printing involving the use of sheets having a small size is improved (Japanese Laid-open Patent Publication No. 2005-194104).
There has been proposed a post-processing device which is employed in printing involving the use of a plurality of sheets having a uniform width but different lengths. The post-processing device is capable of aligning the trailing edges of the sheets when a condition that a preceding sheet is longer than the successive sheet is satisfied (Japanese Laid-open Patent Publication No. 2005-132616).
Conventional post-processing devices have less-restricted space constraints. Thus, it is possible to dispose an output tray, in which sheets having been subjected to a post-process are kept and from which a user takes the sheets, on a downstream of the sheet feed direction to a holding tray. Stated differently, the sheet feed direction to the holding tray can be set to a direction which is the same as the output direction from the holding tray to the output tray.
However, a demand for space-saving in an image forming apparatus recently has been raised. Further, sever space constraints have been put onto a post-processing device. In particular, a demand for downsizing of an inner finisher provided inside the image forming apparatus has been increased. For this reason, the output tray cannot be disposed in a downstream of the holding tray. The output tray, therefore, has to be disposed at a position at which the output direction is orthogonal to the feed direction. For example, if the feed direction corresponds to the right to left direction of the image forming apparatus, then the output tray is supposed to be disposed in the front (or the rear) of the holding tray.
With a post-processing device having a configuration in which the feed direction is orthogonal to the output direction, when a first sheet of paper and a second sheet of paper which are different in width (the size in the width direction orthogonal to the conveyance direction) are sequentially fed into a holding tray for alignment, the second sheet of paper has to be fed into the holding tray after the first sheet of paper is discharged from the holding tray. This is because it is difficult to align the first sheet of paper and the second sheet of paper overlapping with both ends thereof in the width direction shifted from each other. The alignment in such a case is a process to put together one end of the sheets of paper in the width direction. For the alignment, however, an alignment mechanism having a complicated structure is required. Such a complicated alignment mechanism cannot be provided in a compact post-processing device such as the inner finisher.
It may be possible to omit the application of alignment process to the first and second sheets of paper overlapping with both ends thereof in the width direction shifted from each other, and, to discharge the first and second sheets of paper with both ends thereof shifted from each other from the holding tray.
When the sheets of paper are discharged without the alignment process, and when a push-out member for the discharge is pressed onto the sheets of paper to move the same, the push-out member first contacts one of the sheets having a larger width to move the same. At this time, a sheet having a smaller width is probably dragged and moved with the sheet having a larger width. When being dragged and moved, the edge of the sheet having a smaller width often skews with respect to the output direction. In short, the sheets of paper overlapping each other are further misaligned in many cases as compared to the case before the start of output of the sheets of paper.
The misalignment of sheets of paper lowers the quality (appearance) of output. Unpreferably, the skewed sheets outputted may fall from the output tray.
In order to prevent such unpreferable situations from occurring, as discussed earlier, the second sheet of paper has to be fed into the holding tray after the first sheet of paper is discharged from the holding tray in the conventional technologies.
In view of this, in conventional technologies, of mixed-printing type involving the use of a plurality of sheets of paper with different sizes, in printing involving the use of a plurality of sheets of paper with different widths (hereinafter, such printing being referred to as “mixed-printing”), every time when the width of a sheet fed into the post-processing device is changed from the width of a preceding sheet fed thereinto, the post-processing device is supposed to perform output operation. Since feeding the next sheet of paper is stopped until the output operation is completed, the mixed-printing involves a wait time called “wait” between the feed of a preceding sheet of paper and the feed of a subsequent sheet of paper. In short, unfortunately, the mixed-printing is less productive than normal printing which involves the use of sheets of paper of uniform size.